The present invention relates to tools used for tightening threaded joints, and more specifically, to impact wrenches for torque tightening by delivering a number of rotary blows to a threaded joint.
Torque tightening of threaded joints may be most accurately controlled by the energy method. With the employment of impact wrenches, this method consists in that a preset number of rotary blows are delivered to the threaded joint, every blow having the same energy.
At present, it is known to use impact wrenches featuring low blow rate for torque tightening (cf. U.S. Pat. No. 3,952,814). In these impact wrenches, the number of blows is counted by the operator to turn off the impact wrench after a pre-set number of blows are delivered, that is after a predetermined amount of energy is transmitted to the threaded joint.
In case the operator counts the number of blows, human errors may occur associated with rapid fatigue of the operator thus resulting in the need in an objective control of tightening of threaded joints.
Known in the art are impact wrenches having devices for automatically limiting the number of blows to a pre-set number of blows (cf. USSR Inventor's Certificate No. 272173, Cl. B 25 b 21/02) comprising an electrical blow counter which breaks open the supply circuit of the impact wrench motor after a pre-set number of blows are delivered, and the tightening is thus stopped.
It should be, however, noted that the electrical blow counter complicates the electric circuit of the impact wrench because a control circuit is required in addition to the power circuit of the impact wrench. Disadvantages of this counter are also associated with insufficient reliability and durability of a contact setter which is subjected to impact loads during operation, and with difficulties encountered when different types of drive are used for impact wrenches (e.g. pneumatic or hydraulic drive will require two different supply sources, whereas with the employment of a high-frequency low-voltage drive, a number of auxiliary devices are required due to increased current in the power circuit, to prevent burning of the setter contacts, while the size and weight of the impact wrench should remain within the limits acceptable for hand-held power tools).
A device for automatically limiting the number of blows used in a pneumatic wrench (cf. German Pat. No. 2,119,273 Cl. 87a, 13) comprises a mechanical adder. This device includes an inertial mass mounted on the rotor shaft for axial movement and acting on a spring-loaded pusher which causes rotation of the adder shaft by means of an overrunning clutch. The adder shaft is connected, by means of bevel gears, to a longitudinal shaft having a freely mounted toothed wheel which is connectible to the longitudinal shaft by means of a friction clutch engageable under the action of compressed air fed from a starting device of the tool. The toothed wheel of the longitudinal shaft meshes with a setting gear having a pin engaging a leverage system connected to a cut-off valve after the gear is turned through a pre-set angle.
This device may only be used in pneumatic impact wrenches. In addition, it has involved force transmitting connections lowering the reliability of operation of the impact wrench.
In any case, disconnection of the drive of an impact wrench for interruption of delivery of blows to a threaded joint substantially complicates the construction of impact wrench and lowers its reliability.